Twenty-eight quake lakes are at risk of bursting, according to Chinese state media agency Xinhua.

But the one at Tangjiashan - on the Jianjiang river above the town of Beichuan - is the most precarious.

Aftershocks and heavy rainfall make the mammoth round-the-clock task faced by engineers and troops on the steep valley slopes even more hazardous.

The delicate, tortuous work involves heavy machinery gingerly shifting debris from the dam, and engineers blasting dynamite to carefully punch holes in the mountain of rubble and soil - although experts warn this risks further destabilising the structure.

Nearly 160,000 people in the disaster zone have already been evacuated in case the Tangjiashan quake lake bursts.

"This is the most challenging terrain possible to effect an aid and rescue operation," said another geologist, Dr Stephen Edwards, from Benfield University College London's Hazard Research Centre.

"It is topographically hellish, a logistical nightmare. These dams are essentially weak, loose piles of debris under huge pressure from the river water building up behind.

"It's a very vulnerable situation. One more aftershock could suddenly transform this incoherent mass into a devastating wall of liquid slurry.

Troops risk being buried by more landslides or the quake lake

"It would rush off downstream, bulldozing and burying everything in its path."

It is estimated the water will reach the top of the 82 metre (270 ft) high dam at Tangjiashan within two weeks.

Troops and engineers are racing to carve a 500 metre (1,640 ft) channel out of the landscape and divert the water towards the Fujiang river.

They aim to complete the giant sluice and begin draining the 300 million cubic metre capacity lake within 10 days.

"Once the water begins to flow over the top of the dam there's nothing you can do to stop it," said seismologist Dr Alex Densmore, of Durham University's Institute of Hazard and Risk Research.

'Catastrophic failure'

He said it could then be just a matter of time until the dam of landslide debris suffers a "catastrophic failure".

That part of central China is seamed with sedimentary rock - limestone, sandstone and mudstone - which is much weaker than the crystalline rocks, such as granite, found in the European Alps, according to Dr Densmore.

Tens of thousands are fleeing the disaster zone

Much of it has already been undermined by many large earthquakes in the "tectonically very active" area over the past 10,000 years, he said.

He predicted further huge aftershocks, possibly up to magnitude 7, rattling the quake lake valleys.

And in a sobering reminder to those in Sichuan province, he added: "There are a large number of active faults in the area, and we know very little about most of them.

"So the likelihood of another large earthquake in the region is no less than it was before 12 May."

Little wonder then that Premier Wen Jiabao says he regards draining the swelling quake lakes at China's ground zero as the nation's most urgent task.

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